It is well known that the efficiency and the exhaust emissions of an internal combustion engine are dependent upon the operating temperature of the engine. In general, and within limits at higher operating temperatures the efficiency is increased and the volume of emissions is decreased. However, it is known that excessive metal temperatures can reverse the trend and greatly reduce the durability of the engine. Accordingly, it is desired to operate the engine at a high temperature consistent with safe metal temperature in steady state or prolonged running conditions. However, brief excursions to a higher maximum temperature due to transient loading or other operating conditions are permissible without unduly affecting engine durability. It is further desired to bring the engine to its optimum operating temperature immediately after start-up and to prevent overheating after engine shut down. For this purpose, it is desired to provide an engine cooling system utilizing a circulating liquid coolant with a coolant control system which adapts the rate of system heat transfer capacity to the engine heat rejection under various operating conditions so that an optimum operating temperature is maintained without blanket boiling during engine running and without after-boil after engine shut down. Blanket boiling occurs when the boiling point of the coolant is exceeded by the metal temperature and the heat flux is such that vapor is produced at such a rate that a film or blanket is formed over the metal surface. This has the undesirable effect of greatly increasing the resistance to heat transfer with a resulting danger of overheating the engeine. After-boil can also produce a damaging condition; it occurs after engine shut-down due to heat storage or a sudden reduction in local pressure in the engine which causes vigorous boiling of the coolant. This blows the coolant out of the coolant passages in the engine head and restarting the engine without coolant in the head may damage the engine head. In order to meet present day requirements, especially in passenger cars, such a cooling system must provide the afore-mentioned performance characteristics with a small number of low cost components of reduced weight and size and with a minimum of wasted power.
The conventional engine cooling system, as used in present day passenger cars, utilizes a circulating liquid coolant in primarily a convection heat transfer mode between the engine and a heat exchanger or radiator. The most common liquid coolant is a mixture of water and ethylene glycol. Typically the fluid is circulated by a circulating pump driven by the engine which runs proportional to engine speed. The heat transfer capacity of the radiator to the surrounding atmosphere is enhanced by ram air or by a fan which may be driven by the engine or by an electric motor which is controlled in accordance with radiator temperature. In order to provide a fast engine warm-up and for providing a degree of temperature regulation during other operating conditions, a thermostat is provided in a radiator inlet passage which is closed until a predetermined temperature is reached.
The influence on cylinder head temperatures of parameters such as cylinder head material, coolant composition, pressure, temperature and velocity has been investigated and reported by Finlay et al in Factors Influencing Combustion Chamber Wall Temperatures In The Liquid-Cooled, Automotive, Spark-Ignition Engine, Procedures on the Institution of Mechanical Engineers, Volume 199, No. D3, page 207, 1985. According to this report, each of the parameters was systemmatically varied and its influence on combustion wall chambers was measured. The results, according to the authors, suggest that nucleate boiling can play an important role in the transfer of heat from cylinder head to the coolant.
In the prior art cooling systems are known which utilize a variable speed electric motor for driving the circulating pump. This is described in the Gueyen U.S. Pat. No. 4,557,223 granted Dec. 10, 1985. In the system of this patent, the motor speed is controlled as a function of the coolant temperature and as a function of engine intake air pressure. Also, a variable speed radiator fan driven by an electric motor is operated at a speed which is a function of the coolant temperature.
Also, it is known in the prior art to provide a cooling system for an internal combustion engine wherein a liquid coolant is boiled by the heat rejected by the engine and the vapor is used as a vehicle for removing heat from the engine. Such a cooling system is disclosed in the Hayashi U.S. Pat. No. 4,545,335 granted Oct. 8, 1985 and by the Hirano U.S. Pat. No. 4,549,505 granted Oct. 29, 1985.
A general object of this invention is to provide an improved cooling system for internal combustion engines, and especially adapted for automotive vehicles, which overcomes certain disadvantages of the prior art.